Cenozoic
The Cenozoic sediments of Prydz Bay are extensively described in results
from ODP Leg 119 (Barron, Larson, et al., 1989, 1991). Hambrey et al.
(1991) recognized three major sequences composed chiefly of diamictites
(Fig. 8). The upper sequence (PS.1 of Cooper et al., 1991a) is up to 250 m
thick and flat lying (thickest under Four Ladies Bank) but thin to absent
beneath Prydz Channel, Svenner Channel, and the Amery Depression. The
second sequence lies unconformably beneath the upper one at Sites 739
and 742 and consists of steeply prograding foresets of massive and
stratified diamictite with some evidence of slumping (Fig. 8) (PS.2A,
Cooper at al., 1991a ). The lowermost sequence is gently inclined massive
clast-poor diamictite and bedded diamictite and poorly sorted mudstone.
This sequence is richer in kaolinite than the overlying units. The
lowermost sequence was dated as middle Eocene to early Oligocene, the
upper sequence at Sites 739 and 742 was dated as late Miocene, Pliocene,
and Pleistocene. Holocene siliceous, muddy ooze overlies the diamictite in
places (Domack et al., 1991).

Hambrey et al. (1991) interpret the massive diamictites as mostly
water-lain till formed by rainout from the base of glacial ice close to the
grounding zone. They interpret some intervals with deformed bedding,
particularly in the dipping, foreset units as debris flows formed from
glacial debris with minor amounts of remobilized turbidites and
ice-rafted sediment. Hambrey et al. (1991) describe some massive units
with preferred orientation of clasts as possible subglacial till units, an
interpretation supported by evidence of overcompaction of some intervals
in the upper, flat-lying sequence (Solheim et al., 1991).

Hambrey et al. (1991) interpret the succession along the Leg 119 transect
as indicating a history of earliest glaciation of the shelf in the Eocene to
early Oligocene reworking preglacial Eocene sediments and depositing the
lower, gently dipping sediments on the shelf. The ice then moved onto the
shelf as a floating tongue and prograded the shelf edge in the early
Oligocene. A major expansion of ice that Hambrey et al. (1991) regard as
the largest pre-Quaternary expansion occurred during the late Oligocene to
early Miocene.

Seismic data through the Leg 119 drill sites indicate that the flat-lying
upper sequences pass seaward into foreset beds that prograde the
continental shelf edge (Fig. 8; Cooper et al., 1991a, 1991b). Farther west
where Prydz Channel crosses the shelf, the topsets are thin to absent and
deposition has been concentrated on the upper slope in a trough mouth fan
called the Prydz Channel Fan (Fig. 9). Landward of Four Ladies Bank, the
topsets pinch out and the Amery Depression is covered with a thin layer of
till and glaciomarine clayey silts and sands and siliceous muddy ooze
overlying Cretaceous and older sediments. Leitchenkov et al. (1994)
suggested that the shelf prograded more or less evenly across the bay
until some time from the late Miocene to Pliocene. A readily mappable
erosion surface (pp12, Surface A of Mizukoshi et al., 1986) marks the
development of Prydz Channel and the start of Prydz Channel Fan
sedimentation at this time. The fan has been the major depocenter since
then although the Four Ladies Bank has continued to receive topsets of till
(Fig. 9).

The Prydz Bay continental slope and rise are underlain by thick (more than
6000 m) drift sediments, some in elongated ridges aligned along the
margins of deep channels, others having no clear correlation with
channels, but all of them elongate approximately orthogonal to the
continental margin (Kuvass and Leitchenkov, 1992). The seismic geometry
of these drifts suggests that they have been deposited as a result of the
interaction of downslope mass flow and strong bottom (contour) currents.
By analogy with other drift deposits on the Antarctic margin (Rebesco et
al., 1997; Shipboard Scientific Party, 1999), the drifts are composed of
alternating clastic- and biogenic-rich intervals that reflect alternations
of glacial and interglacial conditions. Such records can be compared to the
proximal records of the continental shelf and upper slope to understand
the relationship between oceanographic conditions and the advance and
retreat of the ice sheet.

The most conspicuous sediment drifts are developed in the western part
of the Cooperation Sea between Wilkins and Wild Canyons and are referred
to as the Wilkins and Wild Drifts (Fig. 3). Kuvaas and Leitchenkov (1992)
recognized two major seismic unconformities (P1 and P2). Additional data
and reinterpretation have allowed the mapping of a third surface younger
than P1 and P2. Surface P1 within these sediments marks the transition
from a lower homogeneous part of section, with mostly irregular
reflectors, to an upper, heterogenous one in which a variety of
well-stratified seismic facies are present. More distal data suggests that
P1 may be as old as Cretaceous. Surface P2 marks a change to submarine
canyons and to related channel and levee deposits and chaotic seismic
facies. Kuvaas and Leitchenkov (1992) interpret this transition resulting
from the onset of continental glaciation in the Eocene or the arrival of
grounded ice sheets at the shelf edge in the early Oligocene, as indicated
by ODP Sites 739 and 742 (Barron et al., 1991). This sedimentation change
produced thick, prograding foresets above the P2 unconformity beneath the
Prydz Bay outer shelf (Kuvaas and Leitchenkov, 1992).

Surface P3, above P2, represents the base of deposits containing abundant,
well-stratified sediment drift facies, including sediment waves.
Sediment wave geometry implies that strong, westerly flowing bottom
currents played a significant role in drift formation. The changes at this
level could have been related to initiation of the ACC after the opening of
Drake Passage around the Oligocene/Miocene boundary or may relate to a
major ice expansion during the Oligocene or Miocene.

Quaternary to Modern Sediments
The parts of Prydz Bay shallower than 690 m are extensively ploughed by
iceberg keels (O'Brien and Leitchenkov, 1997) and so are covered with a
layer of disturbed sediment. Surface sediments and their diatom floras
are described in Harris et al. (1998). Areas deeper than this have
undisturbed Quaternary sections. The Amery Depression inshore from
grounding zone wedges in Prydz Channel is floored by fluted subglacial till
and draped in places by clayey diatom ooze deposited since ice retreat in
the last 12,000 years (Domack et al., 1998; O'Brien et al., 1999). The tills
are dark gray pebbly sandy clays with high magnetic susceptibility in
most of the Amery Depression but in the southwestern corner of the Bay,
in the Lambert Deep, tills are brown-red with low susceptibility derived
from pre-Cretaceous red bed sediments in the Lambert Graben (Domack et
al., 1998). Prydz Channel seaward of the grounding zone wedges, is floored
by smooth seafloor and iceberg scours draped by gray clays and diatom
ooze. These draped iceberg scours are probably relict features from LGM
low sea levels.